Known examples of methods and apparatuses for coating a photosensitive material onto a substrate include a multi-slide hopper (U.S. Pat. No. 2,761,791) and a multi-layer curtain coating method (Japanese Patent Publication No. 239/88 and Japanese Laid-Open (Kokai) Patent Application No. 74761/79).
Japanese Laid-Open Patent Application No. 27464/87 disclosed a material for a coating apparatus for applying a photosensitive material.
Japanese Laid-Open Patent Application No. 95170/87 discloses a material for a coating apparatus for applying a magnetic material.
As is clear from the above inventions with respect to a photosensitive material, a magnetic recording material and an information recording material, the coating apparatuses are severely required to be manufactured highly precisely and also not to be corroded by the coating liquid, in order to maintain the uniformity of the product and the stabilization of the production.
However, the conventional coating apparatuses suffer from the following problems:
(1) With respect to any type of conventional hopper, the number of blocks is increased with a multi-layer construction, which results in an increased weight of the hopper body. With such a multi-layer construction, a movable bed for setting the hopper accurately in a coating position is increased in weight so as to withstand such a construction, and as a result a drive device is required to produce a very large power.
(2) The hopper is transferred by a carriage, a crane or the like for the purpose of a periodical maintenance (particularly, washing), and therefore such facilities are also required to have an increased strength and to produce a large power in accordance with the overall weight of the hopper equipment. This much increases ancillary facility costs.
(3) When the hopper is subjected to a periodical washing, the blocks are disassembled and re-assembled. Even a single block has a heavy weight, and a crane or the like is used to transfer the hopper. However, in order to maintain a high precision of the hopper and also to prevent damage to the hopper body, the operator is required to have much skill and to take an extreme care in the disassembly and assembly, which results in increased man-hour.
(4) Because of a thermal deformation of the material of the hopper, the conditions under which the hopper is used are limited. More specifically, one of the effects caused by such a thermal deformation is that the distribution of a clearance of the hopper slot portion is increased, so that the distribution of a coating amount in the direction of the width of the substrate is increased. As a result, the performance of the product is varied at the positions of the widthwise slits in the product. Also, particularly in the case of an information recording paper, this may cause the web to travel in a meandering manner immediately after the coating is applied thereto, and the web may have wrinkles. As a result, the web may fail to be uniformly taken up or wound, thus inviting an irregular winding.
To restrain such effects to a minimum and to deal with such problems, a device for controlling the temperature of a coating liquid or a device for controlling a heat retention has conventionally been used. Also, the number of pass rolls and expander rolls is increased so as to prevent a meandering movement of the web and the formation of wrinkles in the web.
Further, in order to ensure the basic performance of the product even at the minimum portion of the coating distribution, the average coating amount is increased.
Another effect caused by the thermal deformation is that the coating stability is adversely affected in the case of a slide hopper and an extrusion hopper.
In the coating operation using a slide hopper or an extrusion hopper, a gap between the distal end of the hopper and a backup roll constitutes an important factor for stabilizing a high-speed coating. However, because of a thermal strain of the hopper, the gap has a large distribution in the direction of the width to adversely affect the stability of the high speed coating. To deal with this, various surface active agents are unduly added to the coating liquid to further improve its coating properties so as to achieve a continuous production even if there is a large distribution in the gap in the direction of the width.
(5) Not only in the coating of a photosensitive material but also in the coating of an information recording material and a magnetic recording material, there are often used a coating liquid and a washing liquid both of which have the pH of not more than 5 or not less than 10. In this case, the problem of corrosion of the material of the hopper is encountered. See, for example, Japanese Laid-Open Patent Application No. 227464/87.
When corrosion develops, stripes are formed on the coating, and the washing properties of the corroded portion is adversely affected, so that a residual coating liquid is mixed with another coating liquid when exchanging the coating liquids, which may adversely affect the performance of another product.
For these reasons, the hopper is periodically disassembled and washed, and it is checked whether or not corrosion develops. If any corrosion is found, a corroded block is re-polished or is replaced by a new one.
(6) Particularly, many of coating liquids of a magnetic recording material are of a highly abrasive nature, and therefore after a certain period of use, the distal end portion of the hopper is worn. In this case, stripes may be formed on the coating, and a stable coating operation may not be carried out.
For these reasons, the distal end portion of the hopper is subjected to a frequent maintenance such as replacement by a new one and the re-polishing.
With the above problems in view, it is an object of this invention to provide a coating apparatus which achieves the following:
(1) to reduce the weights of the hopper and its ancillary facility to thereby reduce the facility cost;
(2) to make the hopper highly precise (and therefore to reduce thermal influences) to stabilize the coating under a wide range of conditions; to obviate the need for an undue addition of additives, such as a surface active agent, to the coating liquid; to apply a uniform coating onto a substrate in the direction of its width under a wide range of conditions (temperature conditions) to provide a product of a more uniform performance; thereby to better stabilize the conveyance of the web after the coating is applied to the web and to improve a bulky winding condition of the product; to stabilize the operation of a slitter when it cuts the product into various sizes to hereby enhance the efficiency of the manufacturing and processing steps to enhance the productivity; and
(3) to improve the durability (chemical resistance and wear resistance) to reduce maintenance costs.